Jesus told his disciples to wait until they were clothed with power from on high. He said:
Wait for the gift my Father promised, which you have heard me speak about. For John baptised with water, but in a few days you will be baptised with the Holy Spirit… you will receive power when the Holy Spirit comes on you, and you will be my witnesses in Jerusalem, and in all Judea and Samaria, and to the ends of the earth.
— Acts 1:4-5,8 (NIV)
So the disciples gathered in the Upper Room to wait and pray (Acts 1:13-14).